Wooden wind turbine blade manufacturing process

ABSTRACT

A wooden wind turbine blade is formed by laminating wood veneer in a compression mold having the exact curvature needed for one side of the blade, following which the other side of the blade is ground flat along its length but twisted with respect to the blade axis.

The Government has rights in this invention pursuant to Contract No.DE-AC04-76DP03533 awarded by the U.S. Department of Energy.

p This is a division of application Ser. No. 379,631 fled May 19, 1982,now U.S. Pat. No. 4,465,537 issued Aug. 14, 1984, assigned to theassignee of the present application.

BACKGROUND OF THE INVENTION

The invention relates generally to the manufacture of rotor blades andmore particularly to the manufacture of a wind turbine blade with a flatbottom air foil.

Wind turbine blade design is critical to the overall performance,reliability and cost of modern wind energy conversion systems. The winddriven rotor typically comprises two or three radially extending bladesconnected at the center by a rotor hub which in turn is drivinglyconnected to an electrical generator or alternator. Thirty foot diameterrotor designs are capable of generating six kilowatts of electricalenergy at moderate wind speeds. Each fifteen foot blade comprises acomplex elongated geometry of flat twisted and convex surfaces. Whilewood is an excellent choice for this type of rotor blade because of itsphysical properties and absence of electrical interference, shapingtechniques used in the past have relied on a variety of complex bladecarving machines of the tracer type designed to carve both the top andbottom side of the air foil. These machine require small cutter widthsand multiple passes to accurately carve an air foil. Thus, the machineryis not only expensive but labor intensive to use in the manufacturingprocess. Nevertheless, the resulting blades vary widely in stiffness,weight and strength characteristics from piece to piece. Thisvariability, which can adversely affect rotor performance, results in alower yield thus increasing the cost of an already costly process.

SUMMARY OF THE INVENTION

The general objective of the invention is to manufacture more uniformwooden wind turbine blades at lower cost.

These and other objects of the invention are achieved by a novelmanufacturing process in which thin sheets of wood veneer are laminatedin a compression mold having one side with the exact curvature neededfor the complex convex side of the blade, following which the other sideis ground flat but twisted along the length of the laminated blade byautomated grinding machinery. The preferred process utilizes a drumsander. The twist distribution along the axis of the blades comes fromwarping the flat cut as it moves outward from the root to the tip of theblade. The only labor intensive steps are simple straight band-saw cuts,hole drilling and finishing of the laminated blade. The result is ablade with more uniform stress, weight and strength characteristics frompiece to piece at a reduced manufacturing cost.

In the preferred process, melamine urea glue is applied to the flatsurfaces of the veneer strips and while the veneer is compressed by anair bag press, the glue is hardened and cured by radio frequencyheating.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of the molding process.

FIG. 2 is a perspective schematic view of the molded laminated bladeblank.

FIG. 3 is a plan view of the blank of FIG. 2 with trim lines indicatedin dashed lines.

FIG. 4 is an end view of the automatic grinding apparatus.

FIG. 5 is a side view of the apparatus of FIG. 4.

FIG. 6 is a plan view of the finished blade.

FIG. 7 is a cross-sectional view of the finished blade taken in lines7--7 of FIG. 6.

FIG. 8 is a blade section overlay schematic representation from tip toroot illustrating the twist schedule.

DETAILED DESCRIPTION

The preferred manufacturing process is aimed at producing wind turbinerotor blades for a wind turbine of the type illustrated in copendingU.S. patent applicaton Ser. No. 351,896 filed Feb. 24, 1982 entitled"Wind Turbine Rotor Control System" assigned to the assignee of thepresent application and incorporated herein by reference. The preferredprocess of manufacture involves forming the curved laminate in acompression mold, removing the excess wood from the edges with a bandsaw, forming the twisted flat bottomed air foil with an automaticsanding machine, followed by the usual manual hand finishing anddrilling of the completed blade.

FIG. 1 shows the molding process in which elongated rectangular sheetsof 1/16" birch veneer 10 are wetted with adhesive by a conventionalmetered roller applicator (not shown) and stacked on the concave femalepart 12 of an elongated mold. When making a typical 15-foot blade, 32sheets of veneer are required. The curvature of the surface of thefemale mold half 12 is designed to match the air foil curve shown, forexample, in FIG. 8. This curved surface can be as complex as required.The upper mold half 14 carries a male molding surface which isapproximately parallel to the female mold surface but is not critical incontour or dimension. The opposing faces of the mold halves 12 and 14are lined or faced with respective thin flexible sheets of aluminum 16and 18 as shown. Aluminum strips 16 and 18 are connected to a radiofrequency generator 20. The upper mold half 14 is forced downward by anair bag press or hydraulic press, if desired, compressing the veneerlayers. While compressed, the glue is heated and cured by radiofrequency energy transmitted through the laminate by way of the aluminumstrips 16 and 18. The resulting laminated blank 24 is illustrated inFIG. 2. The upper convex veneer surface 24A ultimately forms the curvedside of the rotor blade. The remainder of the blank is machined asillustrated in FIGS. 3-5.

As shown in FIG. 3, the blank 24 is straight cut with a band-saw alongthe dashed lines 26 to define the contour of the edges of the rotorblade. This simple operation can be done by hand.

Next, the flat side of the rotor blade is machined by the apparatusshown in FIGS. 4 and 5. An elongated horizontal worktable 30 supports awooden or preferably fiberglass jig 32 having a convex surface like thatof the mold 12 comprising a long channel parallel to the length of theworktable 30, in which the blade blank 24' trimmed as shown in FIG. 3 islaid convex (good) side down. The blade axis a as used herein and shownin FIG. 3 is defined by the length of the blade from the larger root endwhich is connected to the rotor hub to the outboard tip of the blade.The object of the machinery shown in FIGS. 4 and 5 is to grind the uppersurface of the blade blank 24' flat at a continuously varying angle withrespect to the horizontal along the blade axis a, like the surface of aslightly twisted ribbon.

A wheeled carriage 34 includes a pair of metal plate side panels 36 and38 rigidly interconnected by several crosspieces 39, only one of whichis shown in FIG. 4 for illustration. Each side wall 36, 38 carries a setof three pulley wheels 40 which captively engage upper and lower V-rails42 and 44 respectively mounted in parallel on both side of the table 30as shown in FIGS. 4 and 5. Between the jig 32 and upper V-rails 42 are apair of linear guide or cam rails 46 and 48 extending along the lengthof the table 30 along either side of the blank jig 32. The upper flatsurface of cam rail 46 with a curved profile, as shown in FIG. 6 isengaged by a roller 50 journaled in cam follower assembly 52 which ismounted for vertical translation in a slotted guideway 54 formed in theside panel 36. On the other side a similar roller 56 is journaled in camfollower 58 vertically slidably mounted in a similar guideway in theother side panel 38. Drum 60 is rigidly mounted on shaft 62 for coaxialrotation therewith. A sandpaper belt 61 is mounted on the drum surface.Drum shaft 62 is journaled in a pair of self-aligning bearings 64 and 66in pillow blocks 68 and 70 respectively slidably mounted in theguideways of side panels 36 and 38 above the respective cam railfollowers 52 and 58. The two pillow blocks 68 and 70 have rigid verticalextensions 72 and 74 which are pivotally interconnected by a yoke link76 above the drum 60 parallel to the drum shaft 62. Link 76 is pivotallyconnected to the end of piston 78 of a pneumatic double acting cylinder80 affixed to a rigid superstructure 82 supported by the side panels 36and 38. The best location of the connection point of the piston 78 andyoke 76 can be determined by trial and error. The drum shaft 62 isrotated by means of pulley 84 mounted to the end of the drum shaft 62extending out of the pillow block 70. Pulley 84 is belt-driven by amotor (not shown) mounted for travel with the carriage 34.

The pneumatic cylinder 80 acts as a spring applying constant forcedownward on the pillow blocks 68 and 70. When desired the cylinder canbe used to raise the pillow blocks and the drum 60 from the work piecefor inspection or replacement of the sandpaper belt 61. Because of thedownward force acting on the pillow block 68 and 70 there need be norigid link between the pillow block and its respective cam follower 52or 58. Instead, by design, the pillow block and associated cam followerare movable with respect to each other to allow shims to be placedbetween them to adjust the height of the drum shaft 62 above the workpiece 24'. Locomotion for the carriage 34 can be provided by anadditional pneumatic or hydraulic cylinder and cable arrangement or anyother appropriate means for rolling the carriage 34 along the carriagerails 42 and 44. In operation, the cam rails 46 and 48 cause elevationand tilting of the drum axis b in accordance with roller axes c and d,from the horizontal 72 as necessitated by the twist schedule of theflat-bottomed air foil as shown for example in FIG. 8.

As the carriage 34 travels down the length of the blade blank 24',excess wood is removed by the sandpaper 61 leaving behind a smoothsurface like a twisted ribbon. The work is completed typically inseveral passes using sandpaper belts of the same or varying grade. Thusat any point along the blade axis a, the blade surface transverse to theaxis is absolutely flat but angled with respect to the horizontal inaccordance with the curved profile of the cam rails 46 and 48. The crosssectional view of the finished blade shown in FIG. 7 illustrates theunique construction of the resulting blade. Note that the convex surfaceof the blade is formed by a single sheet of veneer which was molded tothat shape in the compression mold of FIG. 1. The other side of theblade is cut flat across the laminate.

To complete the blade, the leading and trailing edges 90 and 92 as shownin FIG. 7 are hand-sanded. Mounting holes are drilled in the blade root94 of FIG. 6 and the blade is spray-coated with a polyurethane or othersuitable finish.

The foregoing process retains all of the desirable aspects of woodenrotor blades made of inexpensive renewable available materials whileminimizing the usual problems associated with finishing. The laborintensive carving process even using sophisticated tracing machinery hasbeen eliminated resulting in substantial reduction in labor andmachinery cost. An added bonus is derived from the fact that the volumeof scrap wood in mass production is drastically reduced. The inventionpermits rough and finish carving operations to be designed completely asautomatic molding and machining processes. The use of a single moldedside and a flat twisted bottom formed by two guide rails permits analmost infinite variety of configurations to be automatically machinedmerely by changing one mold half and inserting custom guide rails. Thusthe procedure is inherently adaptable to blades of a variety of lengthsand sizes without sophisticated tracing machines. Moreover, theresulting laminated product has improved mechanical properties andreduced variability in stiffness and weight from piece to piece.

The foregoing detailed description is intended to be merely illustrativeof the procedure and apparatus used in this process. Different types ofmolding equipment and adhesive systems may be used in place of the onedescribed herein. While birch veneer is preferred, other types of woodmay be found to be suitable. Only one implementation of the drum sandingmachinery is shown in FIGS. 5 and 6. The details can be modified asneeded. For example, the carriage 34 can be fixed and the workpiecealong with the guide rails moved through the carriage. This equivalentrelative motion between workpiece and carriage might well be preferableon larger workpieces requiring a larger drum and more horsepower. Theseand other variations of the illustrated process and machinery can bemade by those skilled in the art without departing from the spirit orscope of the invention as defined by the appended claims and equivalentsthereto.

What is claimed is:
 1. A laminated wooden rotor blade having aflatbottomed air foil, comprisinga two-sided tapered laminate composedof compression bonded parallel layers of wood having a convex side in apredetermined contour to which all of the layers of wood areapproximately parallel over the entire length of the blade and a flattwisted side, the surface of which at any point along the length of theblade is rectilinear in cross section but of varying orientation alongthe length of the blade according to a predetermined twist schedule,said flat side cutting across the laminae of said blade to reveal theparallel edges thereof.
 2. The blade of the foregoing claim 1, whereinsaid convex surface is formed by a single continuous outer lamina. 3.The blade of claim 1, wherein said laminae are predominantly woodveneer.
 4. The blade of claim 1, wherein said laminae are predominantlybirch veneer.
 5. The blade of claim 1, wherein said laminae are bondedtogether by melamine urea.
 6. The blade of claim 1, wherein said laminaeare bonded together by applying adhesive to the lamina and heating thelaminate under compression.
 7. The blade of claim 1, wherein saidadhesive is melamine urea.
 8. The blade of claim 7, wherein said heatingis accomplished by radio frequency heating.